Some electric utility customers, most commonly commercial and industrial customers, are billed two separate charges on their electricity service bill: a consumption charge and a peak demand charge. The consumption charge reflects the total amount of energy that the customer uses over the billing period. In contrast, the peak demand charge reflects the highest, or peak, amount of power demanded by the customer within the billing period. In practice, utility companies usually average power demand over recurring “demand intervals” (e.g., every 15 minutes), and then use the highest demand interval average within the billing period to calculate the peak demand charge.
For customers that face a high peak demand charge each billing cycle, it can be economical to install an onsite energy generating (EG) system (e.g., a battery-based EG system) that performs “peak shaving.” This means that the EG system discharges energy during intervals of high site load, thereby offsetting energy consumption from the utility grid and reducing, or shaving, the site's peak power demand. Due to the positive impact peak shaving has on lowering energy bills, improvements to methods of peak shaving are desired.
Conventional peak shaving methods for controlling the flow of energy to/from such systems include various features, such as ratcheting, dead band, roll-off and dispatch calculation. Ratcheting is a feature that adapts a control method to a changing load profile by increasing a target peak value when site power demand rises above the target peak value even after an offset from an energy storage system. Dead band is a feature that maintains a buffer above the target peak value where the target peak value is only ratcheted when the site power demand exceeds the current target peak value plus the dead band. Roll-off is a feature that is utilized to increase the life of the energy storage device by adjusting the amount of power that is allowed to be discharged from the energy storage system based on its state of charge (SoC). Dispatch calculation is a feature that builds upon aspects of the previous three concepts. The dispatch calculation feature constantly attempts to lower a power demand value that is based on a historical, rolling window, where the window is based on the utility-defined demand interval.
These existing methods for controlling the flow of energy to/from such systems to achieve peak shaving generally have a number of shortcomings. For instance, some peak shaving methods generate control signals to discharge an energy storage device at the same frequency at which the load level is monitored. Generating the control signal at the same frequency as monitoring the load level causes the energy storage device to frequently alter the discharging amount. Such frequent alteration of the discharging amount decreases the useable life of the energy storage device, i.e., has high “cycling cost,” because of the stress on the energy storage device. Furthermore, not only are such peak shaving methods high in cycling cost, but they can be “unstable” given the constantly-changing discharge pattern.